Multi-pass absorption cell

Abstract

Laser absorption spectroscopy using multi-pass absorption based on multi-planar light conversion is a very sensitive chemical sensing technique for determining the molecular composition and concentration of samples, especially gas samples. In order to obtain a long optical path length while keeping the detector small, many trace gas sensors depend on a multi-pass absorption cell in which the light beam is reflected a plurality of times. The novel method of the invention consists in the use of a multi-planar light conversion (MPLC) phase plate as a reflector in such a multi-pass absorption cell.

Description

Technical Field

[0001] The present invention relates to multi-pass absorption cells based on multi-planar light conversion for use in spectroscopy and, in particular, to the design and fabrication of such cells. Background Art

[0002] Laser absorption spectroscopy is a very sensitive and selective chemical sensing technique in which the molecular composition and concentration of a sample is determined by measuring its absorption spectrum with a laser source. Since the absorbance A of a sample is proportional to its concentration and the optical path length L, as described by the Beer-Lambert law, increasing L allows detection of lower molecular concentrations. This is particularly relevant for gases, which generally have lower absorption per unit length, i.e., lower attenuation coefficients, than solid samples. In order to exploit the long optical path length while maintaining a small form factor for the detector, many trace gas sensors rely on multipass absorption cells, in which the light beam is reflected multiple times to maximize the interaction length in a relatively small volume. The best known multi-pass cell designs are described by JU White, "Long Optical Paths of Large Aperture", J. Opt. Soc. Am. 32(5), 285 (1942) and by D. Harriott, H. Kogelnik, and R. Kompfner, "Off-Axis Paths in Spherical Mirror Interferometers", Appl. Opt. 3(4), 523 (1964). Both designs are based on spherical mirrors.

[0003] Various novel configurations have been introduced over the years to improve the path length to volume ratio and performance. JB McManus et al. in "Astigmatic mirror multipass absorption cells for long-path-length spectroscopy", Appl. Opt. 34 (18), 3336 (1995) introduced a variation of the Herriott design based on astigmatic mirrors and achieved a cell with a path length of 100 meters in a 3 liter volume and a cell with a path length of 36 meters in a 0.3 liter volume.

[0004] Recently, Krzempek et al. (CW DFB RT Diode Laser-Based Sensor for Trace-Gas Detection of Ethane Using a Novel Compact Multipass Gas Absorption Cell", Applied Physics B 112, n°4 (September 2013): 461-65) demonstrated a densely patterned multipass cell using two spherically aberrated mirrors. 3 , that is, in a volume of 0.27 liters, the optical path length in the cell is 57.6 meters long.

[0005] B. Tuzson et al. proposed and demonstrated a more compact design based on a single reflective annular surface in “Compact multipass optical cell for laser spectroscopy” Opt. Lett. 38(3), 257 (2013), which can be used in only 40 cm 3 (0.04 L) volume provides optical path lengths up to 7.1 m.

[0006] Multipass cells are challenging to manufacture. In particular, scattering or any stray light on the mirror surfaces may hinder the practical use of this type of cell for absorption spectroscopy by causing interference fringes, as described by B. Tuzson et al., supra. Existing solutions are based on large mirrors and require significant bulk and adjustments, as described in Krzempek et al., supra and references therein.

[0007] The present invention uses a novel approach to achieve a compact, long optical path, multi-pass cell based on multi-plane light conversion by using multi-plane light conversion (MPLC).

[0008] Multi-plane light conversion (MPLC) is a low-loss beam shaping process that allows any desired unitary transformation of optical modes to be performed, as described by JF Morizur et al. in "Programmable Unitary Spatial Mode Manipulation," Journal of the Optical Society of America A 27, No. 11 (November 1, 2010): 2524.

[0009] MPLC is ideal for multi-beam systems because the beams can be shaped simultaneously. MPLC can be implemented in both transmission and reflection configurations. A particularly attractive implementation of MPLC for multi-pass absorption cells relies on multiple reflections between reflective phase plates and mirrors, as disclosed in many patents and patent applications such as US10382133B2, WO 2018134534A1, WO 2016037850A1, and Guillaume Labroille et al., entitled "Characterization and Applications of Spatial Mode Multiplexers Based on Multi-Plane Light Conversion", Optical Fiber Technology 35 (February 2017): 93-99. The paper discloses that MPLC can perform arbitrary unitary transformations. As shown in Figure 2 of the paper, this is illustrated by demonstrating the injection of light from 10 separate single-mode fibers into the orthogonal modes of a multimode fiber. MPLC is implemented using light injected into a phase plate from a linear array of single-mode fibers and emitted into a multimode fiber after bouncing 14 times between the phase plate and a reflector. The shape of the phase plate defines the light transformation and is optimized to achieve mode conversion from the 10 fundamental modes of a single-mode fiber to one of the modes of a multimode fiber.

[0010] These methods result in small volumes, which is advantageous for many applications. In addition, reflective MPLC systems avoid dispersion and absorption due to propagation through refractive optics. The present invention improves on this technology to achieve long optical path gas cells in small volumes.

[0011] US10382133 B2 describes an MPLC as an optical phase shift component for phase shifting and modifying the intensity of a light beam injected into an optical fiber. The component is inserted at a certain position in the optical fiber. It uses two reflectors and multiple beam optical paths between the reflectors. An optical phase shift structure, such as a reflective phase mask with a structured surface, and ultimately a reflector, is effective at each reflection of the light beam and gradually separates the light beam into faster and slower propagation modes. The faster mode undergoes one or more reflections more than the slower mode and is thereby decelerated. The fast mode and the slow mode are combined again and then transmitted in a multimode optical fiber in which each mode has a different propagation speed. However, this design must be improved to be used in a gas cell. Summary of the invention

[0012] The MPLC system allows to modify the optical modes of 50 single-mode fibers in a bundle and inject them into 50 higher-order modes of a multimode fiber with minimal crosstalk and low loss. Considering that the modes are characterized by their direction, shape, phase front and position, and considering that the MPLC can be designed to arbitrarily convert any mode into any other mode, one can reapply MPLC using the following properties. Instead of placing multiple single-mode fibers at the input, one places two fibers and a reflector orthogonal to the optical axis of the fiber. At the exit, one places a reflector orthogonal to the exit optical axis. Instead of modifying the mode shape, one changes the mode position and direction on the exit reflection plane in such a way that for each exit beam, there is an accepted reflected beam that will travel in the opposite direction towards the reflector placed close to the input fiber. At this point, applying the same design, the exit beam on the input side of the aiming reflector will be reflected and injected into the acceptable mode of the MPLC, which will travel back to the original exit side. This process continues until the final pass through the MPLC is performed, at which point the MPLC is designed to inject light into the exit fiber.

[0013] Considering a centimeter-sized MPLC with 10 reflections and 50 times the reflected optical path length, one obtains a multi-pass cell with an effective length of 5 meters, to compare with current commercial cells, such as those described by JB McManus et al. and B. Tuzson et al. cited above. Furthermore, existing commercial cells are machined, whereas MPLCs can be manufactured using planar semiconductor manufacturing processes, which have the advantage of parallelization, i.e., many units are manufactured simultaneously on a single wafer. Therefore, the volume price of the described novel embodiments can be significantly lower than the machined version. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings show:

[0015] Figure 1 is a schematic representation of a multi-pass gas cell based on MPLC;

[0016] Figure 2a is a schematic top view of a free-space coupled multi-pass gas cell based on MPLC;

[0017] Figure 2b yes Figure 2a Isometric view of the pool;

[0018] Figure 3a is a schematic top view of a fiber-coupled multi-pass gas cell based on MPLC;

[0019] Figure 3b yes Figure 3a An isometric view of the pool;

[0020] Figure 4a is an example of a three-row square reflection pattern on a pool reflector;

[0021] Figure 4b is an example of a similar pattern of three rows of hexagons;

[0022] Figure 5a is a schematic top view of a fiber-coupled multi-pass gas cell based on MPLC;

[0023] Figure 5b yes Figure 5a Isometric view of pool. DETAILED DESCRIPTION

[0024] Specific embodiments of the present invention are described below. It is understood that the embodiments described herein are merely examples and that other embodiments may be utilized by those skilled in the art without departing from the scope of the present invention as defined in the claims.

[0025] Based on the above Labroille paper, it is understood that using the MPLC in reverse, i.e., injecting into the output, provides an inverse unitary transformation, thereby injecting many modes from the multimode fiber and converting them back to 10 separate single modes. A possible embodiment of a spectroscopic cell can be provided by placing a second MPLC before the first MPLC in such a way that the length of the multimode fiber is reduced to zero. The output end of this second MPLC is also 10 single mode fibers, and one can connect the first to the second, the second to the third at the input, the third to the fourth at the output, and so on. Light in this configuration will travel through the device 10 times, performing 14 reflections in each MPLC.

[0026] This embodiment is not optimal because it uses two MPLCs and requires injection into the fiber and back into the MPLS at every complete trip through the system. A first improvement would be to use a single phase plate with 28 reflections by adopting the same phase plate design on a twice as large plate and reproducing the original pattern on the first half and its mirrored version on the second half. Then, another improvement would be to change the orientation of the exiting modes so that they face each other in pairs and are re-injected into the MPLCs through plane mirrors placed at the positions of the fiber array, where only two fibers remain at the input, the first and the last, and at the opposite exit the plane mirror covers all fiber positions.

[0027] In the above embodiments, one converts the injected single mode to the mode of the multimode fiber and then back to single mode. This is a useless conversion and may be suboptimal. In order to implement a multipass cell related to spectroscopy applications, one would like to optimize the system in a different way.

[0028] Three features or properties appear to lead to optimized embodiments.

[0029] First, the phase plate and mirror should provide the highest reflectivity at the operating wavelength, because the light will bounce multiple times between the phase plate and the mirror. The higher the reflectivity, the lower the accumulation of reflection losses.

[0030] Second, and independent of the first, only the injection mode and the exit mode from the laser are optimized boundary conditions. The injection mode may vary slightly with manufacturing variability affecting the far-field energy distribution and shape. The exit mode should be easily collected by the detector, in fact, it should cover the detector and be as small as the detector can be made. This situation maximizes the signal-to-noise ratio and minimizes nonlinearities.

[0031] Third, the intermediate unitary transformation that results in the intermediate mode has no additional restrictions on the shape other than accepting the laser mode and exiting it to the detector. However, it is necessary for the application to achieve that functionality, i.e., if the input wavelength changes, typically by one wavenumber, or if the temperature changes, or the physical alignment of the laser to the system, it can still provide that functionality.

[0032] To optimize the second and third properties, one can define a quality factor that can be used as an optimization function for an automated design search such as a genetic algorithm or similar technique. The goal is to be able to scan a small wavelength region in order to observe changes in the absorption of the gas present between the mirrors of the MPLC and avoid that these changes in attenuation are hidden by unrelated fluctuations. Such fluctuations may occur as a change in the total transmittance of the system due to wavelength changes or because there is crosstalk between modes that have bounced different times. In addition, acoustic vibrations may create interference or signal loss. To evaluate the signal-to-noise ratio, one can simulate the output signal with a given mode for the alignment of the laser, the mirrors toward each other, the wavelength and the temperature. One can then average the output power and its variance. The mode of alignment of the laser and the mirror must be chosen to reflect the range of errors introduced by the acoustic pressure. In addition, the temperature range mode must reflect the accuracy of the range where a temperature controller of sufficient cost will be able to maintain the MPLC and the flowing gas, and the wavelength mode must explore the full wavelength range necessary to complete the spectrum. A sufficiently large number of realizations of the various modes must be explored to ensure that there is no aliasing due to undersampling of the modes. One criterion is that doubling the number of samples changes the signal-to-noise ratio by less than the difference in the signal-to-noise ratio of another configuration for the same number of modalities.

[0033] This optimization maximizes performance, but this may be at the expense of very difficult alignment, temperature stability, or acoustic quietness. To optimize the manufacturing parameters, calculate the mechanical accuracy that can be achieved at a given cost, as well as the temperature stability. Re-evaluate the pool of solutions obtained from the first step of achieving a sufficiently high signal-to-noise ratio, this time using the quality factors of the parts that achieve acceptable performance as the final selection of candidate designs to sample the modes of manufacturing alignment.

[0034] Figure 1 The overall structure of a multi-pass gas cell based on MPLC according to the present invention is shown. The cell consists of a reflective MPLC phase plate, a reflector and a sealed housing equipped with gas inlet and outlet and optical windows. The purpose of the housing is to separate the sample gas under characterization from the ambient atmosphere and to control the gas pressure in the cell. The sealed housing must be equipped with at least one optical window to allow the input beam to enter the cell and the output beam to leave the cell.

[0035] A first embodiment of the multi-pass gas cell according to the present invention is as follows Figure 2a and 2b A single beam cell is depicted in FIG. 1 . The sealed enclosure is not shown. In this embodiment, the input laser beam propagates in free space. The beam undergoes multiple reflections between the phase plate and the mirror, which creates a long optical path in a small volume. The phase plate is designed to compensate for the inherent divergence of the beam due to diffraction to keep the diameter of the reflection constant after reflection. If the output beam needs to propagate over a long distance, the phase plate can be designed to collimate the output beam, or to focus the output beam at a short distance from where the detector of the cell is located.

[0036] The optical path length L in the cell is greater than or equal to

[0037]

[0038] where d is the distance between the phase plate and the mirror, N is the number of reflections on the mirror, and a is the angle between the beam and the phase plate normal. To maximize the ratio of the optical path length to the volume of the cell, the distance between subsequent reflections on the mirror is usually chosen to be much smaller than d, and thus the optical path length can be approximated by

[0039]

[0040] For example, in a cell with a distance d = 10 cm between the reflector and the phase plate and N = 20 reflections on the reflector, the optical path length is approximately equal to L = 4.2 m.

[0041] The phase plate of this first embodiment is designed to be compatible with the laser beam propagating in free space. To achieve this, a reflective phase plate is used. The desired spatial phase profile This is achieved by etching a corresponding height profile z(x,y) into a substrate (e.g. a glass plate). A highly reflective coating, usually a metal film, is then deposited on the surface to maximize its reflectivity.

[0042] In order to reproduce the beam diameter and beam divergence of the input beam at the output of the cell, in a first embodiment, the N phase profiles corresponding to the N reflections on the phase plate are N identical concave spheres or paraboloids. This arrangement produces a periodic sequence of free-space propagation over a distance 2d followed by a focusing mirror of focal length f. For such a periodic system to support a stable ray trajectory that repeats after N periods, the focal length must be chosen to satisfy the following condition

[0043]

[0044] In the case of a spherical reflector, the focal length A of the reflector is related to its radius of curvature R, which is R = 2f. Therefore, the radius of curvature should satisfy the following condition:

[0045]

[0046] A phase plate consisting of N spherical mirrors satisfying this condition reproduces the diameter and divergence of the input beam at the output of the cell.

[0047] In a second embodiment of the present invention, Figure 3a and 3b As schematically shown in FIG, a single light beam is delivered to a multipass cell via an optical fiber and is collimated by a collimating lens attached to the end of a fiber optic patch cable.

[0048] The great design and fabrication flexibility of MPLC phase plates allows the generation of dense multi-row reflection patterns on the mirror. By vertically stacking M rows of multiple reflections, such as Figure 4a and 4b For M=3, the optical path increases in proportion to the number of rows as shown below:

[0049]

[0050] For example, in a cell with a distance d = 10 cm between the reflector and the phase plate, N = 20 reflections per row on the reflector, and M = 3 rows, the optical path length is approximately equal to L = 12.6 m.

[0051] The design and fabrication flexibility of MPLC phase plates allows the generation of multiple rows of reflection patterns on the mirror arranged in a hexagonal grid (also called a triangular grid). This is the densest packing of circles in two dimensions.

[0052] When the application requires the detection of multiple gases with non-overlapping absorption spectra, the tuning range of a single laser is insufficient and several lasers need to be integrated in a single sensor cell.

[0053] Multipass cells require very precise control of the position and angle of incidence of the input beams to achieve the specified performance. Therefore, coupling multiple laser beams into a single multipass cell is challenging. External beam combining optics are required to make the beams parallel and co-linear before entering the cell. This is typically achieved using polarization beam combining and / or spectral beam combining with dichroic mirrors or diffraction gratings.

[0054] Multi-plane light conversion has been shown to be a powerful technique for beam combining, see for example the two references above by G. Labroille et al. By converting multiple single-mode fibers into orthogonal spatial modes, their output beams can be efficiently multiplexed into a single multimode fiber.

[0055] The phase plate of this second embodiment will be similar to the phase plate defined in connection with the first embodiment.

[0056] In another embodiment of the present invention, multiple non-collinear beams are propagated together into a MPLC multi-pass cell and combined at the output of the cell onto a detector. Figure 5a and 5b In this multi-beam cell configuration, shown in Figure 1, the MPLC phase plates and mirrors fulfill two different functions: a) creating a long optical path in a small volume, and b) combining multiple input beams. In contrast to conventional multi-pass cells, it is not necessary to combine the beams before entering the cell. Figure 5a As shown, the light beam is delivered to the cell by a fiber optic bundle and collimated by a microlens array.

[0057] In order to accept M parallel, non-collinear beams at the input and direct them to a single detector at the output, the MPLC phase plate of this embodiment can be constructed, for example, as follows. Except for the last one, each individual phase profile consists of M stacked spheres or paraboloids, whose radius of curvature is determined to be the same as the single beam pool. The final phase profile consists of M stacked flat surfaces that are angled in such a way that the M individual beams are manipulated to intersect at a single point where the detector is placed. This is just one example, but there are other combinations of phase profiles that can be used to implement such a pool.

[0058] In the case of gas sensors utilizing semiconductor lasers, such as near-infrared diode lasers or mid-infrared quantum cascade lasers, by attaching or integrating the laser chip and its collimating optics instead of the optical fiber and its collimating optics at the entrance of the cell, it is possible to reduce the size of not only the multi-pass cell, but also the size of the entire sensor including the light source, gas absorption cell and photodetector.

[0059] The same is true for multi-gas sensors that require multiple lasers. Instead of a fiber bundle delivering light from several laser sources, a semiconductor laser array chip containing multiple emitters with different wavelengths can be mounted or integrated at the entrance of the cell and collimated by a microlens array.

[0060] In summary, a novel multipass cell structure based on reflective MPLC phase plates and mirrors is presented. This structure is compatible with both free-space coupling and fiber coupling of laser sources. By generating very dense reflection patterns, including a hexagonal grid pattern, which is the densest arrangement of circles, it allows the creation of long optical path lengths in a small volume.

[0061] The disclosed MPLC multi-pass cell structure provides a unique possibility of coupling multiple spatially separated laser beams without the need for previous beam combination. The beam combination is achieved by the cell itself, and the beams are generated to a single detector at the exit. This multi-beam configuration can be achieved with a fiber bundle used to deliver the beams to the cell and a microlens array for collimation, or in the case of semiconductor lasers, with a laser array chip mounted at the entrance of the cell and a microlens array for collimation.

[0062] MPLCs can be manufactured using a planar manufacturing process that has the advantage of parallelization, i.e. multiple cells are manufactured simultaneously on a single wafer. Therefore, the volume price of manufacturing MPLC-based multi-pass cells can be significantly lower than the price of current machined cells.

Claims

1. A multi-pass gas absorption cell comprising at least two reflecting devices arranged to increase the total optical path length of a laser beam within a housing or chamber containing a gas to be analyzed, characterized in that At least one of the reflective devices is a reflective multi-plane light conversion (MPLC) phase plate.

2. The gas absorption cell according to claim 1, wherein: To analyze the gas at multiple wavelengths, several spatially separated laser beams of different wavelengths entering the chamber are converted into orthogonal spatial modes, which are reflected by a reflective MPLC phase plate inside the chamber and measured by a single detector.

3. The gas absorption cell according to claim 1, wherein: The chamber contains at least one optical mirror, as well as an inlet and an outlet for a gas and at least one optical window for the laser beam.

4. A gas absorption cell according to any preceding claim, wherein: The MPLC phase plate exhibits a spatial phase profile compatible with laser light propagating in free space, which is achieved by generating a corresponding height profile of the reflective surface of the MPLC phase plate.

5. The gas absorption cell according to claims 1 to 3, wherein: The MPLC phase plate exhibits a spatial phase profile compatible with laser light propagating in an optical fiber, the spatial phase profile being achieved by generating a corresponding height profile of the reflective surface of the MPLC phase plate.

6. The gas absorption cell according to claim 4 or 5, wherein: The spatial phase profile is produced by etching the desired height profile into the surface of a MPLC phase plate, preferably making several MPLC phase plates on a single glass wafer using planar fabrication techniques, and depositing a metal film onto the surface to increase its reflectivity.

7. A gas absorption cell according to any preceding claim, wherein: The N phase profiles corresponding to the N reflections on the MPLC phase plate are provided by N identical concave spherical or parabolic reflectors on the surface.

8. The gas absorption cell of claim 7, wherein the N mirrors are arranged in stacked rows of multiple mirrors, resulting in an increased optical path length.

9. The gas absorption cell according to claim 3 or 4, wherein: A single light beam is delivered to the cell through an optical fiber having a collimator at its end facing the cell.

10. The gas absorption cell according to any of the preceding claims, the multi-planar light conversion (MPLC) phase plate being a glass structure produced by a planar manufacturing process, preferably by making several phase plates on a single wafer.

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